In addition, most studies used surface electrodes which may cause confounding by various factors including pressure, skin moisture, electrode contact and abrasion of the stratum corneum.
Owing to the absence of original information, data for survival were extracted from eligible studies based on univariate analysis without adjustment for age, gender, and other risk factors (e.g., dietary history and genetic predisposition to disease), which may cause confounding bias.
Such sources may cause confounding, and though we adjusted for annoyance from these sources, some residual confounding may be present, particularly from road traffic and railway noise.
Duration of breastfeeding and the risk of asthma could have common determinants, which may cause confounding of the relation between breastfeeding and asthma.
The study doesn't say whether the participants were from similar majors or a wide variety of majors, but differences in fields of study—especially since professors in certain fields (especially the sciences) often require two spaces in research papers may also cause confounding.
The point estimates (hazard ratios) differ greatly between the analyses conducted by Horsdal et al. (17) and ourselves when compared with the analysis by Khalangot et al. (18), likely because Khalangot et al. adjusted for few variables, many of which may have caused confounding.
Choice of endpoints may also cause confounding of results.
These unmeasured factors may cause residual confounding, which was not taken into account.
The lack of this kind of information may cause serious confounding bias.
Lacking of the information for the data analysis may cause serious confounding bias.
